Overlap-welded member, automobile part, method of welding overlapped portion, and method of manufacturing overlap-welded member

Abstract

The present invention provides an overlap-welded member in which an overlapped portion including plural steel sheet members is joined at a spot-welded portion, in which at least one of the plural steel sheet members contains martensite, and the spot-welded portion includes: a nugget formed through spot welding; a heat-affected zone formed in the vicinity of the nugget; the softest zone having the lowest Vickers hardness in the heat-affected zone; and a tempered area formed between a central portion of the nugget and the softest zone and made out of tempered martensite having Vickers hardness of not more than 120% in the case where Vickers hardness of the softest zone is 100%.

Description

TECHNICAL FIELD[0001]The present invention relates to an overlap-welded member obtained by joining an overlapped portion including plural steel sheet members at a spot-welded portion, an automobile part including the overlap-welded member, a method of welding the overlapped portion, and a method of manufacturing the overlap-welded member.[0002]The present application claims priority based on Japanese Patent Application No. 2012-178691 filed in Japan on Aug. 10, 2012, the contents of which are incorporated herein by reference.BACKGROUND ART[0003]In recent years, in the automobile field, high-tensile steel sheets have been increasingly used in order to reduce the weight of a vehicle and improve safety against collision.[0004]Furthermore, the level of strength of high-tensile steel sheets has increased year by year, and for example, hot-stamped members having tensile strength of 1500 MPa or higher have been practically used. The hot-stamped member as used herein is a member obtained by...

AI Technical Summary

Benefits of technology

The present invention provides an overlap-welded member, automobile part including the same, method of welding an overlapped portion, and method of manufacturing the same. The technical effect of the invention is improved peel strength of the spot-welded portion.

Problems solved by technology

This shows that, in the case of the hot-stamped member of a 1500 MPa class or higher, a crack is more likely to occur in the nugget because the toughness is small in the nugget.

As described above, in the case of the spot welding of the high-strength steel sheet, it is considered that the peel strength reduces mainly because the toughness reduces with an increase in hardness of the nugget, and thus, fracture (partial plug fracture) is more likely to occur in the nugget.

However, in the case where the high-tensile steel sheet is subjected to resistance spot welding, spattering of molten steel called splash is more likely to occur as compared with a case where mild steel is subjected to resistance spot welding, possibly making it difficult to increase the diameter of the nugget.

However, there is a restriction resulting from equipment such as a limitation of a welding gun in terms of stiffness.

However, deterioration in productivity is inevitable.

Furthermore, if the distance between spots in spot welding is reduced, electric current is diverted to the spot-welded portions that have been already formed, causing a problem in which nuggets cannot be formed in a stable manner.

As a result, stress concentration on the nugget end 12E is not sufficiently improved.

In other words, an effect of improving toughness through tempering cannot be sufficiently obtained, and hence, it is not easy to sufficiently secure peel strength of the spot-welded portion 10.

As described above, with the conventional subsequent energization method, it is not easy to sufficiently obtain the effect of improving the toughness of the spot-welded portion, and there is a problem in which welding time increases, which leads to a notion that this conventional subsequent energization method is not practical.

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